The overall goal of this project is to develop an effective strategy for management of severe graft-versus-host disease (GVHD) yet preserves donor T cell activity. After allogeneic hematopoietic stem cell transplantation (HSCT), donor T cells contribute to the prevention of graft rejection and relapse (GVH effect) as well as immune recovery but cause GVHD. Current strategies for the management of GVHD do not adequately deal with these competing endpoints. It is proposed that alloreactive donor T cells, modified to express the herpes simplex virus/thymidine kinase gene (HSV/TK; suicide gene) can maintain in vivo function (GVH effect) and be induced to "self-destruct" with ganciclovir to prevent the development of severe GVHD. Preclinical studies of HSCT from transgenic mice that express the HSV/TK gene show that GVHD was controlled and graft rejection did not occur after treatment of the recipient with ganciclovir. Since nondividing nonalloreactive T cells were not induced to apoptose, reconstitution of immunity was improved. However studies in transgenic mice have not reflected the clinical experience to date. In clinical trials, human T cells must be transduced with retroviral vectors after ex vivo expansion with a mitogenic CD3 monoclonal antibody. In these trials, there has been a significant loss of alloreactivity which negated any potential benefit and prevented any conclusions regarding the effectiveness of this strategy for controlling GVHD. In this proposal, studies will be conducted on T cells transduced with viral vectors in a preclinical dog model of HSCT from DLA-haploidentical donors. Functioning alloreactive T cells are required for engraftment and potentially fatal GVHD is predictable in this model. In aim 1, recipient-specific cytotoxic T lymphocytes (CTL) genetically-modified (GM) with retroviral vectors to express the HSV/TK gene will be added back to T-depleted marrow to assess alloreactive function. Since T cells may lose alloreactivity when activated for retroviral transduction, lentiviral vectors will also be studied in this model since activation is not required and both CD4 and CD8 T cells are transduced. In aim 2, the effectiveness of ganciclovir on the induction of apoptosis of GM T cells and the subsequent control of GVHD will be assessed. By preserving the effects of the donor T cells but preventing the development of severe GVHD, the morbidity and the mortality of allogeneic HSCT will be decreased and it will be possible to extend the "curative" benefits of allogeneic HSCT more broadly to include patients with nonmalignant diseases.